An air conditioner (indoor unit) that is installed at a high location, such as an air conditioner that is concealed in or suspended from a ceiling of a house, may use, for example, a metal top plate to form the top surface of a cassette body casing. The air conditioner is concealed in the ceiling or suspended from a lower surface of the ceiling by suspending the main body casing and suspending heavy objects such as the heat exchanger, fan, fan motor, drain pump, and switching box from a top plate and then suspending the main body casing with suspension bolts or the like.
An example of a high location installation type air conditioner is shown as a ceiling concealed type air conditioner in FIGS. 41 to 43.
As shown in FIGS. 41 to 43, the air conditioner is formed by setting an air conditioner body 1 in an opening 7 formed in a ceiling C, and attaching a decorative panel 2 covering the opening 7 to the air conditioner body 1. The air conditioner body 1 has a cassette body casing 3. The body casing 3 accommodates a substantially annular heat exchanger 4, a fan (or impeller) 5, a fan motor 9, and a bell mouth 6. The fan 5 is arranged at the central portion of the heat exchanger 4 in a manner that its air inlet side faces downward and its air outlet side faces the side of the heat exchanger 4. The bell mouth 6 is made of synthetic resin and arranged at the air inlet side of the fan 5.
The fan 5 has a large number of blades 5c arranged between a hub 5b and a shroud 5c. A drain pan 8 is arranged below the heat exchanger 4, and an air outlet passage 10 is formed around the heat exchanger 4.
The body casing 3, which has a substantially hexagonal horizontal cross-section, includes a side wall 3a, which is formed from a heat insulating material, and a top plate 32, which covers an upper portion of the side wall 3a. 
The heat exchanger 4 includes a pair of opposing open ends. Two tube plates 4a are respectively arranged on the two open ends. A predetermined partition plate 12 connects the two tube plates 4a to each other.
The top plate 32 of the body casing 3, the two tube plates 4a, the partition plate 12, and a switch box 13 attached to a lower surface of the bell mouth 6 are all made of metal plates. As shown in FIG. 43, the top plate 32 and the switch box 13 are fixed to the top and bottom ends of the partition plate 12 by screws.
The bell mouth 6 has a recessed portion 14, which is for accommodating the switch box 13, and an opening 16 formed in a top surface 14a of the recessed portion 14. A switch box joint 15 formed on a lower end portion of the partition plate 12 is arranged in the opening 16.
A pair of attachment tabs 17 joined to the top plate 32 is formed on two sides of an upper end portion of the partition plate 12 in a manner that the attachment tabs 17 project integrally from the upper end portion of the partition plate 12. The two attachment tabs 17 are fixed to the top plate 32 from under the top plate 32 via screws 18.
A pair of attachment tabs 19 that is joined to lower ends of the two tube plates 4a is formed on two sides of a lower end portion of the partition plate 12 in a manner that the attachment tabs 19 project integrally from the lower end portion of the partition plate 12. An attachment tab 15 connected to the switch box 13 is welded and fixed to a location between the two attachment tabs 19. The two attachment tabs 19 are fixed to the two tube plates 4a from under the tube plates 4a by screws 20. The attachment tab 15 has an L-shaped basal portion 15a that is joined to the partition plate 12 and a attachment portion 15b that is formed integrally with a distal end of the basal portion 15a to extend downward from the distal end of the basal portion 15a. In a state in which the attachment portion 15b extends from the opening 16 and into the recessed portion 14, the attachment tab 15 is fixed to a top surface 13a of the switch box 13 by screws 21.
As shown in FIGS. 41 to 43, the air conditioner includes a drain pump 22, a float switch 23, a drain pump accommodation portion 24 in which the drain pump 22 is arranged, a partition plate 25 partitioning the drain pump accommodation portion 24, and a lid cover 26 of the switch box 13.
The top plate 32, which has a substantially hexagonal shape in correspondence with the shape of the body casing 3 in the air conditioner body 1, includes a hook-shaped rim portion 32c for fitting the top plate 32 to the periphery of an upper end portion of a side wall 31 of the body casing 3.
The top plate 32 has a plurality of main reinforcement ribs 32a that extend radially from a substantially central portion 33 at which the fan 5 and the fan motor 9 are supported to a peripheral portion at which the substantially annular heat exchanger 4 is supported. The main reinforcement ribs 32a are recessed downward and have a predetermined width and a predetermined depth. The peripheral portion of the heat exchanger supporting portion of each main reinforcement rib 32a includes a stepped portion 32b, which extends downward and has a small depth.
The main reinforcement ribs 32a set basic rigidity (deflection characteristics), strength, and vibration characteristics of the top plate 32 at required levels.
In the above-described structure, the distance between the main reinforcement ribs 32a increases at the peripheral portion of the top plate 32. This may accordingly lower the rigidity, strength, etc. of the peripheral portion of the top plate 32. To prevent this, a plurality of sub-reinforcement ribs 34 are arranged between the main reinforcement ribs 32a as shown in FIG. 43. Each sub-reinforcement rib 34 has the desired shape and size set in accordance with an assumed load of the top plate 32. During the design stage, to keep the static deflection of the top plate 32 at a certain value or lower and avoid resonance that would be caused by the rotation produced by the fan motor 9, the primary natural vibration frequency of the top plate 32 is maintained to have a certain value or higher. Further, reinforcement ribs 33a, which are substantially triangular when seen from above, are also arranged at the substantially central portion 33 of the top plate 32 that supports the fan 5 and the fan motor 9. This improves rigidity (deflection characteristics), strength, and vibration characteristics of the supporting portions at which the fan 5 and the fan motor 9 are supported (refer to patent document 1).
The fan and fan motor supporting portion, which is reinforced by the reinforcement ribs 33a, has a circular grooves formed at each corner defined by the base and vertex. Three fan motor attachment portions a, b, and c are formed at the central portion of each groove. The fan motor 9 is suspended from and fixed to the fan motor attachment portions a, b, and c by mounting members 11, which absorb vibrations, and a mounting bracket 9b. The fan 5 is rotatably supported about a rotation shaft 9a of the fan motor 9.
Patent Document 1: Japanese Laid-Open Patent Publication No. 11-201496
In recent years, there has been a demand for lowering the cost of the above air conditioner including the cost of the top plate 32. To reduce the cost of the top plate 32, the entire plate thickness of the top plate 32 may be reduced (to a plate thickness of, for example, about 0.6 to 0.7 mm) from the present plate thickness (of, for example, 0.8 mm). This would reduce the material cost and facilitate the processing of the ribs etc.
However, in such cases, the rigidity and strength of the top plate 32 would decrease, and measures for preventing vibrations when the fan is driven would become necessary. When the top plate is formed to be thinner than it is now, the material cost of the top plate would be reduced, the top plate would easily be deformed, less force would be required to press and form the top plate, and the processing of the top plate would be facilitated.
However, when the thickness of the top plate is reduced, in the case of the prior art structure described above (i.e., the top plate having radial reinforcement ribs), the static deflection would increase and the primary natural vibration frequency would decrease. Thus, level of the prior art top plates would not satisfy the design standards.
Further, there are many reinforcement ribs having complicated shapes. Such reinforcement ribs would not only increase the cost of molds used when pressing the reinforcement ribs but would also increase the tendency of creases, cracks, and warps being formed.